For numerous applications, it is preferable to use a transmission mode that enables simultaneous operation in both directions between the two points, for the sake of better utilization of the lines.
In particular, this solution is adapted to make the interconnection between the units of a central information process subsystem including a large number of units. Since the current trend is to considerably increase the number of processors and memories, the selection of a bus to effect the exchanges of information between all these units is no longer appropriate. It is preferable in this context to use serial links, connecting each unit to all the other units of the system in such a way as to increase the parallelism. Considering the large number of links required in such a system, the choice of bidirectional links is preferable.
Generally, bidirectional transmission necessitates the presence in each station of a transceiver connected to the lines by an adaptation impedance. In the receiving part, means should also be provided enabling handling collisions, that is, the case where the transmitters of two stations connected to one another transmit simultaneously over the common link. These means must under all circumstances permit the detection of the existence and nature of the signals transmitted by the remote station, from the electrical state of the line.
To solve this problem, the principle of compensation in the receiver for the effect produced on the line by the transmitter of the same station when it is transmitting is typically employed. In the case of transmission, this calls for subtraction of the transmission signal from the signal present on the line. The difference obtained is then representative of the signal transmitted by the transmitter of the remote station. Such a solution is described in European Patent Application EP-A-186142 entitled "Two wire bidirectional digital transmission system".